A common occurrence in older persons is compression fractures of the vertebrae, causing both pain and a shortening (or other distortion) of stature. One common treatment is vertebroplasty, in which cement is injected into a fractured vertebra. While this treatment fixes the fracture and reduces pain, it does not restore the vertebra and person to their original height. Another problem is that the cement is injected in a liquid phase so that it may be unintentionally injected outside of the vertebra and/or may migrate out through cracks in the vertebra. This may cause considerable bodily harm.
Another common treatment is kyphoplasty, in which the fracture is reduced, for example by first inflating a balloon inside the vertebra and then injecting a fixing material and/or an implant. The problem of cement migration is reduced, but not avoided, as a lower pressure can be used to inject the cement.
In general, polymeric cements becomes more viscous as the polymer chain grows by reacting directly with the double bond of a monomer. Polymerization begins by the “addition mechanism” in which a monomer becomes unstable by reacting with an initiator, a volatile molecule that is most commonly a radical (molecules that contain a single unpaired electron). Radicals bond with monomers, forming monomer radicals that can attack the double bond of the next monomer to propagate the polymer chain. Because radicals are so transient, initiators are often added in the form of an unreactive peroxide form that which is stable in solution. Radicals are formed when heat or light cleaves the peroxide molecule. For applications in which high temperatures are not practical (such as the use of bone cement in vivo), peroxide is cleaved by adding a chemical activator such as N,N-dimethyl-p-toluidine. (Nussbaum DA et al: “The Chemistry of Acrylic Bone Cement and Implication for Clinical Use in Image-guided Therapy”, J Vasc Intery Radiol (2004); 15:121-126; the content of which is fully incorporated herein by reference).
Viscous cement is advantageous not only in reducing the risk of its leakage, but also, because of its ability to infiltrate into the intravertebral cancellous bone (interdigitaion) [see G Baroud et al, Injection biomechanics of bone cements used in vertebroplasty, Bio-Medical Materials and Engineering 00 (2004) 1-18]. Baroud also suggests that about 95% of the applied injection pressure is required to overcome friction in the cannula. In addition, viscous material may reduce the fracture.
Examples of commercially available viscous bone cements include, but are not limited to, CMW® Nos. 1, 2 and 3 (DePuy Orthopaedics Inc.; Warsaw, Ind., USA) and Simplex™-P and -RO (Stryker Orthopaedics; Mahwah, N.J., USA). These cements are characterized by a liquid phase after mixing and prior to achieving a viscosity of 500 Pascal second. In a typical use scenario, these previously available cements are poured, while in a liquid phase, into a delivery device.
There have also been attempts to reduce cement migration by injecting more viscous cement, for example, during the doughing time and the beginning of polymerization. However, the injection methods suggested require higher pressures for the more viscous material. Also, some types of viscous materials, such as hardening PMMA, have a small workability window at high viscosities, as they harden very quickly once they reach a high viscosity. This has generally prevented very viscous materials and the associated very high pressures from being used. One possible reason is that as pressures increase, the physician is prevented from receiving feedback on the resistance of the body to the injection of the cement. Thus, over-injection can easily occur.
Some fixing materials, such as polymethylmethacrylate (PMMA), emit heat and possibly toxic materials while setting. These may further weaken the bone and possibly cause the cement to loosen and/or the bone to fracture.
It has recently been suggested that some fixing materials, being harder than bone, induce fractures in nearby bones.
It is also known to use bone-like repair materials, such as a slurry of bone chips, which apparently do not induce such fractures. However, injecting such materials is difficult due to their viscosity.
US patents and applications U.S. Pat. Nos. 4,969,888, 5,108,404, 6,383,188, 2003/0109883, 2002/0068974, U.S. Pat. Nos. 6,348,055, 6,383,190, 4,494,535, 4,653,489 and 4,653,487, the disclosures of which are incorporated herein by reference describe various tools and methods for treating bone.
An additional manner to deliver bone cement into the vertebra is using a tamping instrument (U.S. Pat. Nos. 6,241,734 and 6,613,054), comprising a cannula and a rod, which urges the material within the cannula into the bone.
US patent application 20040260303, the disclosure of which is incorporated herein by reference, teaches an apparatus for delivering bone cement into a vertebra.
Cannulae with working sleeves are describe, for example, in U.S. Pat. Nos. 6,241,734 and 6,613,054, the disclosures of which are fully incorporated herein by reference.